1. Field Of the Invention
This invention relates to a parallel multi-junction Superconducting QUantum Interference Device (SQUID), or a Superconducting QUantum Interference Grating (SQUIG), that can be used for a variety of applications involving the detecting of magnetic flux, including applications where it is desired to measure the absolute magnitude of the magnetic field. More specifically, the device of this invention features a novel geometry for a SQUIG which significantly enhances the flux-to-voltage transfer function to yield an improvement in the device sensitivity in its use as a magnetometer, a gradiometer, and in other applications.
2. Description of Relevant Art
In order to understand this invention, it is first necessary to understand the conventional SQUID. A SQUID (Superconducting QUantum Interference Device) is the most sensitive detector of magnetic flux currently available.
SQUIDs combine the two physical phenomena of flux quantization (the fact that the flux .PHI. in a closed superconducting loop is quantized in units of .PHI..sub.0), and Josephson tunneling, or coupling.
A SQUID is, in essence, a flux-to-voltage transducer, providing an output voltage that is related to and is periodic in the applied flux. The period is one flux quantum, or: EQU .PHI..sub.0 .ident.h/2e.congruent.2.07.times.10.sup.-15 Wb
It is typically possible to detect an output signal corresponding to a flux change of much less than .PHI..sub.0.
A SQUID is extremely versatile, being able to measure any physical quantity which can be converted into a flux. For example, magnetic field, magnetic field gradient, current, voltage, displacement, and magnetic susceptibility can all be measured using a SQUID. As a result of this versatility, the SQUID has been used in a wide variety of applications, ranging from the detection of tiny magnetic fields produced by the human brain and the measurement of fluctuating magnetic fields in remote areas, to the detection of gravity waves and the observation of spin noise in an ensemble of magnetic nuclei.
There are two basic types of SQUIDs. See, for example, Clark, in Superconducting Devices, edited by Steven T. Ruggiero and David T. Rudman, Academic Press, 1990, ch. 2, pp. 51-99, for a detailed review of SQUIDs. The dc SQUID consists of two Josephson junctions connected in parallel on a superconducting loop. The dc SQUID is so named because it typically operates with a steady current bias. The other type of SQUID, the rf SQUID, involves a single Josephson junction interrupting the current flow around a superconducting loop and is typically operated with a radio frequency (rf) flux bias.
This invention relates to the geometric parallel of a multi-junction dc SQUID, or a SQUIG (Superconducting QUantum Interference Grating). In order to better teach the invention and to explain its principles of operation, it is necessary to describe SQUIDs in general and dc SQUIDs in particular. It is further necessary to describe the operation, advantages, and drawbacks of a SQUIG.
A number of different types of Josephson junctions, or weak links, have been demonstrated. These include superconductor-insulator-superconductor (SIS)junctions, superconductor-normal metal-superconductor (SNS) junctions, superconductor-semiconductor-superconductor (S-Sm-S) junctions, microbridges, ion irradiated links, and grain boundary junctions. One skilled in the art would appreciate that the superconducting regions used in each type of Josephson junction may be of the same type or may be of different types. For example, one could have a Nb-I-Nb junction or a Nb-I-Pb junction. See Likharev, Rev. Mod. Phys., Vol. 51, p. 101 (1979).
The dc SQUID consists of two shunted Josephson junctions interrupting a superconducting ring as shown in FIG. 1a. Each junction, represented by crosses, , may have a capacitance C ranging from zero to several picofarads, and each junction is shunted by a resistance R. The junctions are resistively shunted to eliminate hysteresis in the current-voltage (I-V) characteristic, which is shown for an unshunted junction in FIG. 1b for .PHI. equal to n.PHI..sub.0 and .PHI. equal to (n+1/2).PHI..sub.0, where .PHI. is the external flux applied to the loop and "n" is an integer. The constant bias current, I.sub.B, (greater than two times the critical current, or 2I.sub.0) maintains a non-zero voltage across the SQUID, which has a non-hysteretic current-voltage characteristic. If the magnetic flux, .PHI., threading the SQUID ring is slowly varied, the critical current will oscillate as a function of .PHI. with a period that is just .PHI..sub.0. The critical current is a maximum for .PHI.=n.PHI..sub.0, and a minimum for .PHI.=(n+1/2).PHI. .sub.0.
The effect of the magnetic field is to change the phase difference between the two junctions. The oscillating behavior arises from interference between the wave functions at the two junctions in a manner analogous to interference in optics--hence the term "Interference Device." At low voltages, the current-voltage characteristic is also modulated because the current contains a contribution from the time averaged ac supercurrent. As a result, when the SQUID is biased with a constant current, the voltage is periodic in .PHI. with period .PHI..sub.0, as shown in FIG. 1c.
Modern dc SQUIDs are invariably made from thin films, with the aid of either photolithography or electron beam lithography. In most, although not all, practical applications, the SQUID is used in a feedback circuit as a null-detector of magnetic flux. A small modulating flux is applied to the SQUID with a frequency usually between 100 and 500 kHz, as indicated in FIG. 2. If the quasistatic flux in the SQUID is exactly n.PHI..sub.0, the resulting voltage across the SQUID is a rectified version of the input signal, that is, it contains only the second harmonic of the reference frequency, as indicated in FIG. 2a. If this voltage is sent through a lock-in detector referenced to the fundamental frequency, the output will be zero. On the other hand, if the quasistatic flux is (n+1/4).PHI..sub.0, the voltage across the SQUID is at the fundamental frequency, as shown in FIG. 2b, and the output from the lock-in detector will be a maximum. Thus, as one increases the flux from n.PHI..sub.0 to (n+1/4).PHI..sub.0, the output from the lock-in detector will increase steadily; if one reduces the flux from n.PHI..sub.0 to (n-1/4).PHI..sub.0, the output will increase in the negative direction, as indicated in FIG. 2c.
A typical modulation and feedback circuit for a dc SQUID is shown in FIG. 3. This circuit is essentially a flux-locked loop. The alternating voltage across the SQUID is coupled to a low-noise preamplifier, usually at room temperature, via either a cooled transformer or a cooled LC series-resonant circuit. An oscillator applies a modulating flux to the SQUID via a feedback and modulation coil. The geometry, dimensions, efficiency, and actual construction of the feedback coil are not critical; the feedback coil's magnetic field should simply be isolated from the pickup loop or gradiometer winding and be uniform over the loop created by the SQUID. After amplification, the signal from the SQUID is lock-in detected and sent through the integrating circuit. The smoothed output is connected to the modulation and feedback coil via a large series resistor R.sub.f. Thus, if one applies a flux .delta..PHI. to the SQUID via the input coil (or otherwise), the feedback circuit will generate an opposing flux of -.delta..PHI., and a voltage proportional to .delta..PHI. appears across R.sub.f. This technique enables one to measure changes in flux ranging from much less than a single flux quantum to many flux quanta.
Applications of the SQUID. One of the simplest instruments using a SQUID is the magnetometer. A pickup loop is connected across the input coil (as shown in FIG. 3), which couples the flux into the SQUID, to make a superconducting flux transformer, as shown in FIG. 4a. The SQUID and input coil are generally enclosed in a superconducting shield. The pickup loop and flux transformer function as a magnetic "antenna," or magnetic "hearing aid," where the flux passing through the pickup loop is coupled into the much smaller loop of the SQUID. An important requirement for optimum sensitivity is that the inductance of the pickup loop should be about equal to the inductance of the much smaller input coil, which typically has between ten and one hundred turns. Since the size of the SQUID is restricted by other considerations, such as the increase of noise with SQUID loop area, this requirement imposes constraints on the maximum size of the pickup loop and, therefore, the overall sensitivity of the magnometer. Magnetometers have usually involved flux transformers made of Nb wire. Magnetometers with typical sensitivities of 0.01 pT Hz.sup.-1/2 have been used in geophysics in a variety of applications, such as magnetotellurics, active electromagnetic sounding, piezomagnetism, tectonomagnetism, and the location of hydrofractures.
An important variation of the magnetometer is the gradiometer. The pickup and input coils for an axial gradiometer that measures .differential.B.sub.z /.differential.z is shown in FIG. 4b. The two pickup loops are wound in opposition and balanced so that a uniform field B.sub.z, such as the z-component of the earth's field, links zero net flux to the flux transformer. A gradient .differential.B.sub.z /.differential.z, on the other hand, does induce a net flux and thus generates an output from the flux-locked SQUID. FIG. 4c shows the pickup and input coil for a second-order gradiometer that measures .differential.B.sub.z /.differential.z.sup.2.
The most important application of the gradiometer is in neuromagnetism, notably to detect weak magnetic signals emanating from the human brain, The gradiometer discriminates strongly against distant noise sources, which have a small gradient, in favor of locally generated signals, One can thus use a second-order gradiometer in an unshielded environment, although the present trend is towards using first-order gradiometers in a shielded room of aluminum and mumetal that greatly attenuates the ambient magnetic noise, In this application, axial gradiometers of the type shown in FIG. 4b actually sense the magnetic field, rather than the gradient, because the distance from the signal source to the pickup loop is less than the baseline of the gradiometer. The magnetic field sensitivity referred to the pickup loop is typically 10 fT Hz.sup.-1/2. There are two basic types of measurements made on the human brain. The first detects spontaneous activity with a classic example being the generation of magnetic pulses by subjects suffering from focal epilepsy. The second type involves evoked response, for example, Romanie et al., 216 Science 1339 (1982), detected the magnetic signal from the auditory cortex generated by tones of different frequencies.
Conventional dc SQUIDs have been used for a number of years to measure extremely small values of voltage, magnetic flux, and magnetic flux gradients at low frequencies. Low T.sub.c superconductors, requiring liquid helium cooling, have been used to form the dc SQUID superconducting loop. The recent advent of high temperature superconducting materials, which allow operation at liquid nitrogen temperatures, has strengthened interest in the dc SQUID.
The most distinctive property of a superconductive material is its loss of electrical resistance when it is at or below a critical temperature. This critical temperature is an intrinsic property of the material and is referred to as the superconducting transition temperature of the material, T.sub.c.
Research into the ability of specific materials to superconduct began with the discovery in 1911 that mercury superconducts at a T.sub.c of about 4.degree. K. Since then, many applications for superconducting materials have been conceived, but such applications could not be commercialized because of the extremely low T.sub.c of the superconducting materials then available. Liquid helium, which is itself expensive, and the complicated refrigerant units required for liquid helium, were cost prohibitive to the use of such low T.sub.c superconductors in many applications.
Until about 1986 the highest temperature superconductor known was Nb.sub.3 Ge having a critical temperature, T.sub.c, of approximately 23.2.degree. K. Before 1987, superconducting devices, even those which employed the Nb.sub.3 Ge superconductor, required the use of liquid helium as the refrigerant-coolant.
In late 1986, Bednorz and Muller disclosed that certain mixed phase compositions of La-Ba-Cu-O appeared to exhibit superconductivity being at an onset temperature, T.sub.co, of about 30.degree. K. Bednorz et al, Z. Phys. B., Condensed Matter, vol. 64, pp. 189-198 (1986). The upper temperature limit of superconducting onset, T.sub.co, of superconductors of a this type crystalline structure is no greater than about 38.degree. K. Liquid helium was still required as the coolant for such a 214 type of material.
A new class of rare earth-alkaline earth-copper oxides was then discovered that are superconductive at temperatures above the boiling point of liquid nitrogen, 77.degree. K. These new rare earth-alkaline earth-copper oxides are now commonly referred to as "123" high-temperature superconductors, in reference to the stoichiometry in which the rare earth, alkaline earth, and copper metal atoms are present, namely a ratio of 1:2:3.
With the discovery of the 123 class of "high temperature superconducting" (HTS) compounds--HTS compounds are those which superconduct at a T.sub.c above the temperature at which liquid nitrogen can be used as a refrigerant--it has become economically possible to pursue many previously conceived applications of the superconductivity phenomena which before were commercially impractical wherein cooling by liquid helium was required. Since they superconduct at temperatures greater than 77.degree. K., the new 123 high temperature superconductors may in practical applications be cooled with liquid nitrogen--a more economically feasible refrigerant. Liquid nitrogen is about 2000 times more efficient to use in terms of cost, both of the refrigerant itself and the associated refrigerant unit design.
Detection of Magnetic Flux with Parallel Josephson Junction Arrays.
The sensitivity of the magnetometer is dependent on the flux-to-voltage transfer coefficient V.sub.101 of the active device, such as the dc SQUID. One way to increase V.sub..PHI. is to produce a device with sharper peaks in a plot of total critical current versus flux for the active device. A dramatic enhancement and sharpening of the peaks in critical current versus flux, resulting from quantum interference, was first predicted by Feynman et al., The Feynman Lectures on Physics, Vol. III, Chap. 21, pp. 21-18 (1965), for an array consisting of many Josephson junctions (JJs) in parallel, by analogy to the enhancement of the peaks in an optical slit diffraction pattern when a double slit is replaced by N slits or, equivalently, a diffraction grating. This enhancement was first observed experimentally in the mid- to late 1960s using superconducting point contact junctions. Zimmerman & Silver, 141 Physics Review 367 (1966); Waele, et al., 40 Physics 302 (1968). Three- and four-junction interferometers were developed for logic and switching applications in the 1970 s. Stuehm & Wilmsen, 20 Appl. Phys. Lett. 456 (1972); Zappe, 27 Appp. Phys. Lett. 432 (1975). More recently, a parallel array of several (3-5) weak links near a control line has been successfully utilized in a high T.sub.c "superconducting flux flow transistor." Hohenwarter, et al., MAG-25 IEEE Trans. Mag 954 (1989); Martens, et al., 65 J. Appl. Phys. 4057 (1989). However, since the 1960s there has been virtually no art except for recent work by the inventors involving the use of parallel JJ arrays, or multi-junction interferometers, for the detection of magnetic flux, and no magnetometer or gradiometer incorporating a JJ array with many junctions in parallel has ever been built. Miller, et al., 59 Appl. Phys. Lett. 3322 (1991). One problem has been that the effects of nonuniformity of the junction critical currents are nonobvious.
Consider the N-loop interferometer, or Superconducting QUantum Interference Grating (SQUIG), illustrated in FIG. 5, in which N+1 junctions are in parallel. In FIG. 5, the Josephson junctions are represented by crosses , .delta..sub.i represents the phase difference between the "wavefunctions," or order parameters on opposite sides of each junction, and .PHI..sub.i represents the magnetic flux passing through each loop. In the low inductance limit, the flux .PHI..sub.i through each loop is equal to the externally applied flux per loop .PHI., but this is no longer the case when the loops have significant, finite self- and mutual- inductances. The total critical current as a function of applied flux .PHI. is therefore nonobvious for the case of finite inductance.
Practical SQUIGs often have significant nonuniformity of the junction critical currents, which must be taken into account in order to meaningfully assess the viability of SQUIGs for magnetic flux detection. For a ten-junction SQUIG with .delta.=0.4, with increasing values of the standard deviation .sigma., are shown in FIG. 6, which illustrates that randomizing the critical currents only slightly reduces the principal maxima in critical current versus flux. This result indicates that nonuniformity of the junction critical currents has a substantially less adverse impact than nonuniformity of the flux coupled into each loop, the latter of which could be minimized by utilizing a suitable geometry designed to minimize nonuniformity of the flux coupled from the input coil to the SQUIG loops.
There has been a long-felt need for a device which would be capable of more accurately measuring magnetic fluxes. More particularly, there has been a need for a device based on SQUID technology which would be able to detect and accurately measure small absolute fluxes or small flux changes.